The excitation system of a synchronous generator is primarily directed to supplying direct current to a field winding of the generator to constantly maintain or regulate an output terminal voltage of the generator. The excitation system inhibits disturbance generated from a power system through the regulation of the terminal voltage to thereby improve stability, and controls transmission voltage or high voltage of a step-up transformer of a power plant to thereby stabilize voltage, thereby serving as a power condenser and a synchronous condenser at a load terminal.
The excitation system can be divided into a controller part that functions to control voltage of the generator and protect and restrict the excitation system of the generator, and a phase control rectifier part that receives a control signal from the controller and then supplies a required field current. The controller part is basically made up of a terminal voltage setter, a detecting and filtering part of the voltage of the generator, and a proportional integral controlling part of the voltage of the generator, and electrically and thermally protects the generator to be controlled and the excitation system itself, thereby allowing the generator to exert the maximum performance within its own capability.
The phase control rectifier part secures excitation voltage from an excitation transformer connected to an exciter or a generator output terminal, converts the alternate current voltage into direct current voltage at a thyristor phase control rectifier, and then supplies the converted direct current voltage to a field system of the generator. From the viewpoint of the capability of the synchronous generator, the excitation system automatically regulates field current in response to a change in active output or reactive output and a change in terminal voltage of an synchronous machine within a range in which the synchronous generator can continuously operated, thereby having to be able to rapidly and stably maintain the generator terminal voltage as a target value. Further, the excitation system performs field reinforcement conforming with instantaneous and short time performance of the generator, thereby having to be able to cope with transient disturbance.
With the development of technologies for semiconductor devices, a high capacity rectifier appeared. Thus, most of excitation systems have recently employed static excitation systems. The static excitation system has a fast control response and high ceiling voltage, and thus is advantageous to improvement measures for transient stability of the system, but may damage dynamic stability. For this reason, in order to inhibit power disturbance of the power system from the excitation system having fast response, a power system stabilizer (PSS) is added to contribute to stable operation of the power system.
The excitation system can be generally classified into three types according to its constituent instrument and controller: a direct current exciter system using a direct current generator, a static excitation system configured of an excitation transformer and a thyristor transformer (rectifier), and an alternate current exciter system configured of an alternate current exciter generator and a diode rectifier. A recent trend shows that the static excitation system, which has a response due to the development of power semiconductor technologies and is favorable for maintenance a lack of a rotational part, is mainly applied. The static excitation system obtains excitation power from an output terminal of the generator, so that, when line contingency occur at the output terminal, the excitation power cannot be stably secured.
The excitation system must constantly maintain the generator voltage in order to supply stable power to the power system, and be able to rapidly restore the voltage when the power system undergoes sharp voltage drop. To this end, the excitation system requires a function for controlling the voltage of the generator, and a function for protecting the generator and its surrounding systems.
In conjunction with the voltage control of the generator, an automatic voltage regulator (AVR) functions to automatically regulate the terminal voltage of the generator so as to be matched with a given setup value despite a change in operation situation of the generator.
Synchronous generator systems to which the AVR is applied are widely used because of robustness against load variation and high reliability, ranging from generators for land plants to emergency generators for buildings and military and marine power equipment. In general, a number of AVRs for controlling the output voltage of the generator have employed analog AVRs designed for the generators. However, the system is complicated due to demands for parallel operation and high-performance control, in addition to difficulty in AVR application and variation in parameters associated with production of various generators.
Recently, due to the performance problem, the conventional AVR has been gradually converted into a digital AVR (DAVR). A variety of controllers for constantly controlling the output voltage of the generator in the DVAR system are studied. However, most of actual plants widely employ a classic proportional-integral-derivative (PID) controller. This is because the PID controller is simple, and is familiar to on-the-site engineers. Nevertheless, the PID controller shows different response characteristics depending on which control gain is selected. As such, it is very difficult to regulate the control gain. If a high control gain is selected in order to reduce a steady state error, there is a problem that high overshoot etc. occurs during a transient response.
In addition to the problem of this control technique, the existing exciter control system has problems with the response of the power converter and the stability of noise of the switching device. As the power converter for controlling the field voltage of the existing exciter, a thyristor control rectifier (TCR) is classically and widely used. However, in the case of the TCR, a three-phase synchronous machine having a rated frequency of 60 Hz, a control signal can be output once per 180 Hz, and thus the response characteristic of a transient state is very slow. Due to noise caused by high-frequency current based on control of this firing angle, a gate generator and a controller cause malfunction.